Method for reducing fixed pattern noise in solid state imaging devices

ABSTRACT

An amplifying circuit comprising an amplifying element and a memory element as well as an element to adjust the signal in an output terminal of the amplifying element to a known level, a measure of the corresponding level in a control terminal of the amplifying element being stored on the memory element.

RELATED APPLICATIONS

This application is a divisional application which claims priority under35 U.S.C. § 121 from a co-pending and commonly-owned U.S. applicationentitled, "Circuit, Pixel, Device and Method for Reducing Fixed PatternNoise in Solid State Imaging Devices," application Ser. No. 08/742,241and filed on Oct. 31, 1996. This application claims benefit ofprovisional applications 60/007,087 filed Oct. 31, 1995, and 60/026,345,filed Sep. 19, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to solid state imaging devices beingmanufactured in a CMOS- or MOS-technology. More particularly, anamplifying circuit and a method for eliminating fixed pattern noise inthe output signal of a pixel or of an image sensor are disclosed.

2. Description of Related Technology

Solid state image sensors are well known. Commonly solid state imagesensors are implemented in a CCD-technology or in a CMOS- orMOS-technology. Solid state image sensors find a widespread use incamera systems. In this embodiment a matrix of pixels comprising lightsensitive elements constitutes an image sensor, which is mounted in thecamera system. The signal of said matrix is measured and multiplexed toa so-called video-signal.

CCD-based camera systems have less noise fluctuations in the imagecompared to CMOS- or MOS-based camera systems. Therefore CCD-basedcamera systems are nowadays preferred in applications wherein a highimage quality is required such as video or still camera applications.Due to the further miniaturization of the CMOS electronics technology,it is possible to realize complex CMOS- or MOS-based pixels as small asCCD-based pixels. It is a further advantage of CMOS- or MOS-based pixelsthat CMOS is a technology being offered by most foundries whereasCCD-technology is rarely offered and a more complex and expensive one.

Of the image sensors implemented in a CMOS- or MOS-technology, CMOS orMOS image sensors with passive pixels and CMOS or MOS image sensors withactive pixels are distinguished. An active pixel is configured withmeans integrated in the pixel to amplify the charge that is collected onthe light sensitive element. Passive pixels do not have said means andrequire a charge-sensitive amplifier that is not integrated in the pixeland is connected with a long line towards the pixel. For this reason,active pixel image sensors are potentially less sensitive to noisefluctuations than passive pixels. Due to the additional electronics inthe active pixel, an active pixel image sensor may be equipped toexecute more sophisticated functions, which can be advantageous for theperformance of the camera system. Said functions can include filtering,operation at higher speed or operation in more extreme illuminationconditions. It remains however a main drawback of active pixel CMOS orMOS image sensors, hampering their use in applications requiring a highimage quality, that their output signal has an additional non-uniformitycaused by the statistical spread on the characteristics of theelectronic components composing the active pixel. An example of suchcharacteristic being subject to manufacturing process variations is thethreshold voltage of MOS transistors integrated in the pixel. If noprecautions are taken, this non-uniformity, called fixed pattern noiseor FPN, is seen as a "snow-like" shade over the image being taken with aCMOS or MOS image sensor with active pixels.

Document U.S. Pat. No. 4,274,113 discloses a solid state imaging devicehaving a sensor portion and a signal processing circuit. Said signalprocessing circuit comprises means to eliminate fixed pattern noise. Inthis device the charge collected on a passive pixel is measured on acommon charge amplifier. The pixel is sampled two times consecutively,and the difference of charges is constituting the signal. The firstsampling is taken with the pixel not connected to the output, the secondsampling is taken with the pixels connected to the output for a shorttime, so that the charge is integrated in the output. This method toeliminate FPN however is not suited for active pixels which have not acharge output, but which have a voltage output or current output orsimilar. Also, as disclosed in the preferred embodiment of U.S. Pat. No.4,274,113, reading charge over a large bus with high capacitancedeteriorates the temporal noise in the final sensor image.

Document U.S. Pat. No. 4,809,074 discloses a solid state imager having asignal processing circuit for reducing noise, in particular FPN. Eachpixel has two switches, which makes it possible to read a pixel chargewith less noise and FPN than in pixels with only one switch. The FPNcancellation is performed by reading twice the charge on the outputnode. Again, the method disclosed in this patent is not suited foractive pixel imaging sensors.

The patent application Ser. No. WO92/16999 discloses a circuit forminimizing the variation in characteristics across different parts of animaging device caused by manufacturing process variations between aplurality of inverting amplifiers in said imaging device. Said circuitcomprises at least one transistor connected in series with a powersupply terminal on each of the inverting amplifiers so as to provide anew reference level for each inverting amplifier whereby the switchingthreshold of the inverting amplifier is controllable by a voltageapplied to a control input connection of said transistor. This methodapplies only for an imaging device with passive pixels.

Document EP-A-0665685 discloses an active pixel image sensor. Thispatent describes a method to cancel non-uniformity of the pixelresponse. This method is based on the fact that all pixels outputs arefed through switches and column/row buses to a common output, withoutpassing trough intermediate (column) buffers. This method is viable, butit requires the output stage of every individual pixel to be powerfulenough to drive the buses/output lines at a high readout speed. Theproposed method furthermore adds additional fixed pattern noise by themultiplexing structures.

SUMMARY OF THE INVENTION

The present invention discloses a device such as an image sensor whichpermits to reduce fixed pattern noise (FPN) which is invariant in time,without introducing noise of other origin. This device comprises amatrix of active pixels as well as electronic components or circuitslocated by preference at the edges or border of the matrix of pixels. Atthe expense of forming an image sensor with an unusual large area, saidelectronic components or circuits can also be integrated in said pixels.Said electronic components or circuits comprise at least one amplifyingcircuit which is common to a group of pixels such as a column or a rowin said matrix. Furthermore, said device has an output line that ispreferably common to said matrix. The amplifying circuit comprises anamplifying element and a memory element that are connected to circuitrythat is provided to change the signal in the output terminal of saidamplifying element to a known level, and to store the correspondinglevel, or a measure thereof, in the control terminal of said amplifyingelement on said memory element. The active pixels are adapted for beingbrought in a state corresponding to an amount of radiation collected onsaid pixel, and can therefore be changed in to a first state. It isrequired that this first state can be compared to a second state that isdifferent. Said first state can correspond to an amount of collectedradiation or light on the radiation or light sensitive element in saidpixel. Said first state can also correspond to the reset state of thepixels or to a non-illuminated or dark condition of the pixel. Saidsecond state can correspond to a non-illuminated or dark condition ofthe pixel, or to an amount of collected radiation or light on theradiation or light sensitive element in said pixel, or to the resetstate of the pixel.

Preferably, the amplifying element is a transistor and more particularlyof the type of metal oxide semiconductor transistors wherein said outputterminal is the source or the drain of the transistor and wherein thecontrol terminal is the gate of said transistor. It also can be a morecomplex amplifier in its own. The memory element is preferably acapacitor or a nonvolatile memory element as used in ROMs, EPPROMs,EEPROMs, or flash EEPROMs.

In a first aspect of the present invention, an amplifying circuit isdisclosed comprising an amplifying element and a memory element, as wellas connections and circuits to adjust the signal in an output terminalof the amplifying element to a predetermined level, a measure of thecorresponding level in a control terminal of the amplifying elementbeing stored on the memory element.

In a second aspect of the present invention, a device for imagingapplications comprising said amplifying circuit and comprising a groupof pixels is disclosed.

In a third aspect of the present invention, a pixel is disclosed,adapted for integration in an imaging device, comprising an amplifyingcircuit with an amplifying element and memory element as well asconnections and circuits to adjust the signal in an output terminal ofsaid amplifying element to a known level, a measure of the correspondinglevel in a control terminal of said amplifying element being stored onsaid memory element. Said pixel has preferably a photo sensitive elementsuch as a photodiode or an infrared photo detector.

In a fourth aspect of the present invention, a method is disclosed foreliminating fixed pattern noise, which is invariant in time, in theoutput signal of an image sensor making use of said device.

The method is as follows:

the readout output signal of essentially each pixel in said image sensorwhen in a first state, is compared, by preference subtracted from, thesignal of the same pixel when in a second state. Only the compared, bypreference subtracted, signal of both states is transferred to a commonoutput line. The two states are to be read out consecutively bypreference. This method further comprises the step that the outputvoltage of the amplifying element or amplifier, that is placed percolumn by preference at the border of the pixel matrix, is forced to aknown output voltage during the read out of the first state, byadjusting an offset through a feedback mechanism. If afterwards theoutput signal of the said pixel in said image sensor when in a secondstate is being read out, the output voltage of the amplifier shiftsproportional to the difference of both states. As a result, the outputsignal of the amplifying element or amplifier is changing essentiallyonly due to the difference in the amount of light or radiation collectedon the light or radiation sensitive element of the pixel between bothstates. Said output signal, therefore, does not include fixed patternnoise of pixels nor fixed pattern noise of the amplifier or amplifyingelement itself.

In a fifth aspect of the present invention, the use is disclosed of saidimaging device and said method in camera systems and in imagingapplications requiring a high image quality. An example of such camerasystem is a video or still camera or a camera integrated in a multimediadevice such as a Personal Computer equipped with video functionality orwith video and speech functionality. The imaging device of the presentinvention is configured as a pixel matrix that is used as a focal planeimage sensor. As is well understood by a technologist in the relevantfield, for this purpose the integrated circuit containing the pixelmatrix and the peripheric circuits are packaged and mounted inside acamera housing with a lens, at the same place where a photographic filmwould be located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of an active pixel containing three transistors,and a photodiode that can be used in the present invention.

FIG. 2 shows an embodiment of the device for imaging applicationsaccording to the present invention.

FIG. 3 shows three amplifiers that can be integrated as components(11),(61),(103),(111),(123) in the device for imaging applicationsaccording to the present invention.

FIG. 4 shows current sources that can be used in the column amplifiersof the device for imaging applications according to the presentinvention.

FIG. 5 shows another embodiment of the device for imaging applicationsaccording to the present invention.

FIG. 6 shows two pixels of a structure that could be part of an arrayshown in FIGS. 2 or 5.

FIG. 7 schematically shows the timing diagram of the method foreliminating fixed pattern noise in a solid sate imaging device accordingto the present invention.

FIG. 8 shows an embodiment (further referred to as ACI) of the presentinvention with the amplifying circuit in connection with aphotosensitive element.

FIG. 9 shows a prior art amplifying circuit.

FIG. 10 shows another embodiment (further referred to as AR) of thepresent invention with the amplifying circuit in connection with aphotosensitive element.

FIG. 11 shows a differential amplifier with similar use as theamplifiers of FIG. 3.

FIG. 12 shows another embodiment (further referred to as ARI) of thepresent invention with the amplifying circuit in connection with aphotosensitive element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the sequel and for the purpose of teaching only an implementation ofthe image sensor as a sensor with nxm pixels in a double metal, singlepoly 0.7 μm CMOS process can be assumed. The size of such image sensorwith 384×288 pixels is 6, 3×5, 7 mm. The dimensions of the lightsensitive elements in the photodiodes of the pixels are 14×14 μm.Photodiodes of this size generate currents of about 1 nA at lightillumination level of 1 mW/cm². It is obvious that these numbersrepresent only one option within the art and that many otherimplementations will be evident for those skilled in the art. An exampleof an active pixel (1) containing three transistors (100),(101),(102)and a photodiode (103) that can be used in the present invention isshown in FIG. 1. Switch (20) and line (6) have the same function as theparts (21),(23),(71),(73), and (6),(56) respectively in the FIGS. 2 and5. A pulsed signal on gate "RESET" will zero the accumulated charges onthe photodiode (103), i.e. its forces the pixels in a "reference state".

A device to readout active pixels or parts of an imager with activepixels is the scheme represented in FIG. 2. In this figure an arbitraryarray of 2 by 2 active pixels (1),(2),(3),(4) is drawn with one columnamplifier (in this text also referred to as the amplifying circuit) (5).Said column amplifier (5) may as well be integrated within each pixel(1),(2),(3),(4), so as to form a more elaborate active pixel in its own,at the cost of area. This integration may be a preferred option forimaging devices and for a sensor device for non-optical applicationswhich are arranged in a fashion which is not row/column wise, forinstance, linear image sensor.

The output signal of an active pixel (1),(2),(3),(4) is via an optionalswitch (21),(22),(23),(24) read out over a line (6),(7) and is passed toa common output bus (8) via an optional switch (25). Non-uniformities ofdifferent signals on the different lines (6) and (7) of different suchcolumn amplifiers (5) are compensated at the output bus (8). Therelative offsets of the input signals (6) and (7), including the offsetnon-uniformity induced by the output transistor (in this text alsoreferred to as the amplifying element) (9) itself, is compensated byadjusting an offset compensation input of a second amplifier (11). Thisadjustment or calibration is carried out while forcing the active pixelthat is being read out, or the signal on line (6) or (7) in general, toa reference state. For adjustment or calibration, switches (27) and (26)are closed. The Source of transistor (9) is thus forced to a knownvoltage (30). This known voltage (30) can be a supply voltage of theimaging device and is preferably common for all columns of the matrix ofpixels. With appropriate circuit components and applied voltages andcurrents, which are straightforward for those skilled in the art, thevoltage at the drain and gate of the transistor (9) will settle to anequilibrium value. One can predict the behavior of such circuits by theuse of analog circuit simulators such as HSpice, commercialized by thecompany Metasoft. The offset needed to accomplish the equilibrium isstored in an analog memory which is represented as a capacitor (10) anda switch (27). Other ways to implement analog memory, known to the manskilled in the art, can be used for this purpose too. For the normalread out operation, the switches (27) and (26) are opened. Switch (28)is closed to allow for a large supply current for transistor (9). Theoptional switch (25) is used for each column amplifier (5) separately toconnect to the bus (8) in sequential or in a random addressed manner.

The multiplexing done in this way through switch (25) may as well beaccomplished in different ways.

The transistor (9) is preferably a MOSFET, but other types of activeelements like bipolar transistors or thyristors, or JFETs are alsoappropriate. The second amplifier (11),(61) in FIGS. 2, 5 respectivelyis generic. Many known types of amplifiers can be used for this purpose,as long as there is provided means to adjust their offset voltage by anexternal voltage or current. Three of such second amplifiers, accordingto the art are shown in FIG. 3, with an offset adjusting means drawn inthick lines. The amplifier of FIG. 11 can be used for the same purpose.Note that the offset adjusting input is a voltage input in each of thesecases. However, the input device is a MOSFET used as current source, soan input current could be applied as offset means directly as well, Notealso that the second input of a two-input amplifier in itself can beconsidered also as an offset adjusting input, and can be used for thatpurpose.

The second amplifier (11) is not necessarily a unity amplifier. It mayinclude also a filter function to improve e.g. signal to noise ratio.The use of a source follower transistor for second amplifier (11) is asimple and straightforward implementation.

The capacitor (10) in FIG. 2 is a means for analog memory. Other typesof analog memory may be used instead, e.g. based on techniques used forthe fabrication of non-volatile memories (ROM, EPROM, EEPROM), known tothose skilled in the art. Analog memory can also be implemented by acombination of ADC and DAC and digital memory.

The current source (12) may be implemented in well- known ways, as e.g.a MOSFET with a voltage at the gate. In this way however, the currentsof different column amplifiers (5) will have different values due tonon-uniformities of the electronic devices. This non-uniformity of thecurrents will have second order effects on the uniformity of the finaloutputs on the bus (8). One can implement means to obtain more uniformor reproducible current sources (12). Those skilled in the art are awareabout many such methods been devised in the past, e.g. for thefabrication of analog-digital convertors or digital-analog convertors.

One method consists in the use of one single current source for all saidcolumn amplifiers in the imaging array. The said single current sourceis applied to all column amplifiers in turn by means of switches.

Another method consists of the calibration of the current sources whichare residing in each column amplifier during those time intervals thatthese currents are not in use: e.g. during the time that switch (28) inFIG. 2 is closed. This can be accomplished by a circuit configuration asdepicted in FIG. 4. The output (I) of this subcircuit is the currentsource (12) depicted in FIG. 2. The switches (42) and (43) serve toconnect and calibrate the MOSFET (44) for a known current level. Anothermethod is the use of circuit components which are not MOSFETs and thussuffer less from non-uniformities, as resistors, Bipolar transistors orJFETs.

An implementation of the imaging device according to another preferredembodiment of the present invention is shown in FIG. 5. A matrix of 2×2active pixels is drawn. Every pixel (51),(52),(53),(54) can, via aswitch (71),(72), (73),(74), impose its output signal to the columnlines (56),(57).

A pixel can be switched in its reference state, e.g. via a "resetsignal" (this is a common but not exclusive way to do such).

During the first of the two states, switches (77) and (76) are closed,and (78) and (75) remain open. The current source (62) delivers a smallfixed current. For this reason, the source of MOSFET (59) comes to afixed potential, and the gate of this MOSFET is forced to an equilibriumvalue. This value is memorized on the capacitor (60). The amplifier (61)at the gate of the MOSFET is not absolutely necessary, it may beshunted.

The possibly unwanted offset of the pixel signal stands now over thecapacitor (60).

Before the second phase, the switches (77) and (76) are opened, and (78)and (75) potentially too are closed. Consequently, on the source of theMOSFET (59) appears the signal that was there already during the firstphase, except from a small fixed shift. This signal is offered to thecommon output amplifier "U" of the imaging device, which does notcontribute to the FPN because it is common to the whole matrix.

During the second of the two states of the active pixel, the signal online (56) changes. But this new signal contains the unwanted offseteither via the capacitor (60) the change of this signal is applied tothe gate of transistor (59), and when (75) is closed, to the outputamplifier of the imaging device.

Many column amplifiers are in parallel on the output bus (58). One canthen scan these amplifiers by activating their respective switches (75).

We draw the attention to the fact that the circuit in FIG. 5 is aspecial case of the circuit in FIG. 2. The combination of (60) and (61)in FIG. 5 is a second amplifier with AC-coupled input. This too is to beconsidered as an amplifier with offset adjustment terminal. The realinput and the offset input are at both end of the said capacitor (60).

In fact the variant of the circuit in FIG. 2, where amplifier (11) isjust a short circuit, thus the said combination of (60) and (61) beingonly a capacitor, should be considered for this reason as trivial caseof an amplifier with offset correction too. This may be sufficient forthe required functionality.

Other embodiments of the present invention are described in the sequel.

The active pixels to be used with this type of readout must obey only toone criterium, that they can be set in two or more different states,corresponding to different amounts of collected light, or to differentsignal levels in general. The signal path that generates the FPN shouldbe in common for both states, i.e. pass through the same amplifying pathin the pixel. Obvious variants are:

Not limited to a simple pixel. An active pixel may be constructed so asto take its signal from different places and times, as e.g. fromdifferent photodiodes that may be shared with other active pixels, orfrom information that is acquired at different times. Such array isshown in FIG. 6 in a most simple form. Two pixels of an array are shown,only relevant parts of the pixels are shown: (81),(82) are photodetectors, (91), (92) are schematic representations of the amplifyingpart of the pixel.

The switches (83), and (84) are closed and open respectively. Either oneof the detectors (81) or (82) is tied to the amplifier (91) (andsimilarly for all other members of the array). The reference state forsuch a reading may be during the time that (81) is tied to (91), and thereading state is then during the time that (82) is tied to (91).Somewhere down the signal path the output of (91) is corrected using theamplifying circuit (5) as shown in FIG. 2. The function that is realizedin this particular way is a spatio-temporal filter where the intensityat one pixel at one time is subtracted from the intensity of anotherpixel at another time. Several pixels in a neighborhood could be tied atthe same node, at the same or different times, thus realizing morecomplex spatio-temporal filters. In principle the rectangles (81) and(82) may represent active pixels themselves, thus creating a hierarchicstructure.

An array may be different from the classic two-dimensional (rectangular)array. E.g. a linear array, a circular, a log-polar, even a random arrayis possible. If the technology allows so, a 3-dimensional array maybecome possible. The array could be made up of other types of detectorsthan optical detectors, as sensors for electro-magnetic radiation,elementary particles, pressure waves (sound), chemicals, . . . .

In FIGS. 2 & 5, the current source (12),(62) may be replaced by aresistor, or it may be combined with switch (28),(78) in one component(note that e.g. both a current source or a switch can be realized with aMOSFET).

As mentioned before, second amplifier (11),(61) may be omitted(shunted). The circuit could be tuned to operate without secondamplifier (11),(61) or it may include also filtering or a voltage offsetwhich may contribute to a better operation (high speed, low noise,operation in required voltage range etc.).

Switch (26), (76) in FIGS. 2 & 5 may be replaced by a current source ora resistance or any device that can keep the source of (9),(59) at acertain voltage that must not be explicitly applied.

In the above disclosure for simplicity it is supposed that the activepixels (1),(2),(3),(4),(51),(52),(53), (54) output is a voltage. Inpractice, an active pixel may output its signal via a transistor, ofwhich the load is common for the bus, and resides outside the pixel. Thetransistor together with the external load element (being a passivecomponent like a resistor, or an active component like a MOSFET), is atransistor amplifier, typically a source follower, an invertingamplifier, or a differential amplifier.

It is obvious that active pixels (1),(2),(3),(4),(51),(52),(53),(54)which output a current instead of a voltage may be used as well. Acurrent may be converted to a voltage over a resistor or a switchedcapacitor.

It is obvious that, wherever MOSFETs are mentioned in this text, othertypes of controlled amplification devices can be thought, as far as theycan realize the same functions, as bipolar transistors or J-FETs.

As the difference between active pixel and passive pixel lies merely inthe presence of an amplifying element in the pixel, the method may beapplied to cancel the FPN of passive pixels too. For this purpose thecharges of the passive pixel can first be converted.

It is possible to realize an FPN-free reading of an active pixel arrayin other ways:

by reading the said two states separately and doing the subtractionoutside the imaging device, in analog or in digital mode afterAD-conversion. Such an approach requires a double readout speed, andsome kind of analog or digital memory.

by using other types of differential amplifiers at the column edges. Thesignals of the two states may be stored on a capacitor, and thedifferential amplifier read the difference. This is a viable way ofreading the active pixel signal, and removing the FPN of the activepixels, but it does not cancel FPN that is generated in the differentialamplifier itself.

A timing scheme of the FPN elimination method according to the presentinvention is shown in FIG. 7. The signals to be expected at terminals ofthe preferred embodiments (see figures hereabove), while reading twopixels on the same column are shown. The shown applied binary signalsare (77),(27),(76),(26),(78),(28) and (75),(25) (which are the same forsimplicity; but in combination with a multiplexer (not drawn) (75),(25)is different from (78),(28), and Reset, which sets the pixel in a resetstate or not (indicated as "first state" and "second state"). Two analogsignals are shown: (6),(56), the voltage of the column line whichreflects the output of the pixel(s) connected to it, and which is theinput of the column amplifier (5),(55); and the voltage at the source oftransistor (9),(59), which is the output of the column amplifier(5),(55). The white arrows indicate the difference of the resultingsignal going from pixel (1) to pixel (2).

Another embodiment of the present invention is shown in FIG. 8. A pixelincorporating a photosensitive element and an amplifying circuitcomprising an amplifying element (101) and a memory element (102) isshown. Furthermore shown are connections and circuits to adjust thesignal in an output terminal of said amplifying element (101) to a knownlevel, a measurement of the corresponding level in a control terminal ofsaid amplifying element being stored on said memory element. A prior artamplifying circuit is shown in FIG. 9.

The pixel shown in FIG. 9 advantageously is used in case thephoto-current of a photosensitive element or a detector is readout by anamplifier with capacitive feedback (108), which integrates thisphoto-current and gives an output voltage that is proportional to thephoto-current and integration time. The integration time is determinedby timing signals, the photo-current and an integration capacitor (108).Another (second) capacitor (106) is needed to allow the voltage (Biasminus zero-Bias) across the photo detector to be controlled in a preciseway. Specific applications require such features for reasons ofphotometric performance. One operational disadvantage of the amplifyingcircuit of FIG. 9 is that, for good suppression of the parasitic effectsat the input, the ratio of the capacitances of both capacitors (106) and(108) should be as large as possible. In some cases, this ratio shouldbe larger than 1000. Yet for some applications, the capacitance of saidintegration capacitor (108) is 1 pF as imposed by the magnitude of photodetector current. Thus, the second capacitor (106) should be 1 nF insuch cases. Such a large capacitor cannot be integrated on chip.

A new charge amplifying circuit making use of the present invention istherefore disclosed in FIG. 8.

In order to overcome the need of a large number of discrete capacitorsin the circuit of FIG. 9, a pixel as shown in FIG. 8 is proposed. Here,an amplifying circuit based on the present invention is presented. Saidamplifying circuit is referred to as the AC-coupled direct injectioncircuit (ACI). An AC-coupled amplifier in the feedback loop (103) isused to adjust the input mode of the amplifying element (101),preferably a MOSFET transistor. The photo-current is integrated on athird capacitor (104). The column amplifier of the present invention isused in combination with a photo receptor forming thus a pixel. As thecapacitors in this embodiment may have small values, the circuit can beintegrated easily on chip, as a single element or as pixel arrays.Another advantage of this circuit is that the variation and drift of thebias voltage across the photo receptor is very low.

Another improvement of the AC-coupled transimpedance amplifier is shownin FIG. 10. This circuit is referred to as AC-regulated transimpedanceamplifier (AR).

Here, the drawback of using a large second capacitor is solved by usingan amplifying circuit according to the present invention. The memoryelement (112) is combined with a switch "reset". An amplifier (111) withan extra offset input is used as the feedback amplifier and at the sametime as the amplifying element. The photo detector's current isintegrated on the capacitor (114).

As a normal transimpedance amplifier configuration, the current isintegrated on a feedback capacitor (114). Yet during reset, thezero-bias voltage is enforced on the amplifier's input (111) and theoffset imbalance is stored on the capacitor (112).

The circuit as shown in FIG. 11 is an example of a differentialamplifier with two negative and two positive input nodes which could beused in amplifier (111).

The circuits of FIGS. 8 and 10 can be combined in one circuit. Suchcombined circuit (AC-regulated direct injection (ARI) circuit) isrepresented in FIG. 12. The memory element (122) is a capacitor incombination with a switch S3. The analog signal on this memory elementis the input on an additional offset correcting input of a secondamplifier (123) which in turn adjust the gate voltage of the amplifyingelement (121).

This circuit combines the following advantages:

precise control of the BIAS-ZERO BIAS voltage over the photo detector,

no need for large capacitor, the amplifier can be integrated on chip incombination with a photo receptor (120).

Advantageously, said pixels can be used in cryogenic readout amplifierspossibly in stressed GeGa detector arrays. For space applications andfor low leakage applications, the Germanium-Gallium (Ge:Ga) detectorsshould be operated at 1.6 Kelvin, at a bias voltage of 20 mV. Theclosely coupled readout electronics should be operated at the sametemperature and maintained this bias with an accuracy of 0.1 mV.

What is claimed is:
 1. A method for reducing noise in a solid stateimaging device having a group of active pixels, said method comprisingthe steps of:reading out the signal of a pixel in a first state whileforcing the output voltage of an amplifying element that is connected tosaid pixel to a predetermined output voltage level, thereby defining acorresponding voltage level on a control terminal of said amplifyingelement and storing said corresponding voltage level or a measurethereof on a memory element that is connected to said pixel and therebydefining, a first voltage at the output of said amplifying element;subtracting said first voltage at the output of said amplifying elementfrom a second voltage at the output of said amplifying element, saidsecond voltage being defined by the output signal of the same pixel in asecond state and said second voltage including the voltage level storedon said memory element, whereby the output voltage of the amplifyingelement shifts proportionally to the difference of the signal of saidpixel in said first and said second state; transferring the subtractedsignal to an output line that is common for said group; and repeatingthis operation for essentially all or part of the pixels of the imagingdevice.
 2. The method as recited in claim 1, wherein said first andsecond state of said pixel are read out consecutively.
 3. The method asrecited in claim 2, wherein said first state or said second statecorresponds to an amount of radiation or light collected on theradiation or light sensitive element in said pixel or to the darkcondition of said pixel or to the reset state of said pixel.
 4. Themethod as recited in claim 3, wherein said second state corresponds to anon-illuminated condition of said pixel.
 5. The method as recited inclaim 2, wherein said first state corresponds to a non-illuminatedcondition of said pixel.
 6. The method as recited in claim 5, whereinsaid second state corresponds to an amount of radiation or lightcollected on the radiation or light sensitive element in said pixel. 7.The method as recited in claim 2, wherein said predetermined outputvoltage level is derived from a supply voltage of said solid stateimaging device.
 8. The method as recited in claim 1, wherein saidpredetermined output voltage level is derived from a supply voltage ofsaid solid state imaging device.
 9. The method as recited in claim 1,wherein said solid state imaging device is a camera system.
 10. Themethod as recited in claim 1, wherein forcing the output voltage of theamplifying element comprises directly adjusting said output on theoutput terminal of the amplifying element and indirectly adjusting saidcorresponding voltage level on a control terminal.